1. Field of the Invention
The present invention relates to a thermosetting covering sheet that can form a hard coating on the surfaces of objects. This sheet can be overlaid onto the surfaces of various objects and then hardened by heat to create a permanent coating for purposes such as corrosion-proofing, decoration, or display, and it is suitable for covering objects with curved or irregular surfaces. The present invention also relates to a method for forming hard coatings using the said sheet.
2 Description of the Prior Art
Coating materials in liquid or powder form are generally applied to the surfaces of objects for decoration, display, or corrosion-proofing, or to impart various other functional characteristics.
When coating materials that contain organic solvents or coating materials in powder form are used, the working environment deteriorates. On the other hand, when water-base coating materials are employed, the working environment may not be adversely affected, but the long period of time required for the drying of such coating materials, or the rapid drying of such coating materials using e.g., heat may entail substantial energy consumption. Moreover, ordinary coating materials require careful adjustment of viscosity or concentration, which involves complex and tedious work.
Therefore, covering sheets composed of thermoplastic resins such as polyvinyl chloride have been proposed as a substitute for conventional coating materials. A substrate such as a steel plate, etc., is covered by effecting the adhesion of this type of sheet onto the surface of the substrate. The use of this type of sheet has no adverse effects upon the working environment, and involves no need to remove solvents.
When covering the substrate with such a sheet, various known vacuum or pressurization methods for applying the said sheets are used in order to improve the adhesion between the sheet and the substrate. For example, a method for effecting close adhesion of a sheet to the substrate by formation of a vacuum is proposed in Japanese Laid-Open Patent Publication No. 53-61486, and a method of effecting close adhesion by applying the sheet to the substrate in a vacuum chamber is proposed in Japanese Patent Publication No. 57-27009. However, because the aforesaid sheets are composed of thermoplastic resins, the hardness of the coating formed on the surface of the substrate is poor, and consequently such coatings are easily scratched.
As a remedy for this shortcoming, methods for forming coatings of high surface hardness by hardening of the sheet after adhesion to the substrate have been proposed. Japanese Patent Publication No. 57-13425 discloses a type of composite sheet consisting of an initiator-impregnated sheet (which is obtained by impregnating a porous sheet base material with a radical initiator) laminated with a sheet containing a polymer and a radical reactive monomer. Japanese Patent Publication No. 60-21630 proposes a self-supporting sheet composed principally of thermoplastic resin and/or rubber, with ingredients that include a powder containing a radical reactive unsaturated compound (which is solid at ordinary temperatures) and a powder containing a radical initiator (which is also solid at ordinary temperatures).
The uniform hardening of the composite sheet described in the aforesaid Japanese Patent Publication No. 57-13425 requires that heating and pressurization be used to effect uniform contact between the components contained in the two layers of the said composite sheet; however, control of the heating and pressurization processes is difficult, and accordingly, formation of a uniformly hardened coating is difficult. Moreover, substances such as paper, woven cloth, or unwoven fabric are to be used as the basic material to be impregnated by the radical initiator. The elongation of this sort of base material is difficult. Therefore, when this sheet is used for covering the surfaces of curved or irregular substrates, wrinkles are easily formed. That is, with use of this type of sheet, wrinkle-free coatings are formed with difficulty on substrate surfaces.
For the sheet described in the aforesaid Japanese Patent Publication No. 60-21630, because the sheet is readily extensible, its application to irregular or curved surfaces should be possible. This sheet incorporates a powder containing a radical reactive unsaturated compound and a powder containing a radical initiator, but they do not come into direct contact within the thermoplastic resin layer. By heating of the sheet, the two powders and the resin are fused together, thereby effecting hardening of the sheet. Consequently, the quality of the sheet is dependent on the grain size of the powder and the uniformity of mixing of this powder. In the process of the manufacture of this sheet, because the two powders start to react when brought into contact at high temperatures, each powder must be refrigerated when the crushing and mixing are performed. Uniform mixing of the two powders is difficult, so the reaction induced by heating the sheets so obtained tends to be nonuniform. As a result, nonuniform covering coatings are liable to be formed, which detracts from the external appearance of such coatings.
In Japanese Patent Publication No. 61-61873, a sheet made of a composition containing a photocrosslinking acrylic polymer and a photosensitizer is proposed. The substrate is covered with this type of sheet and then the sheet is hardened by irradiation. However, if this type of sheet is used to cover objects with irregular or curved surfaces, then the sheet cannot be uniformly irradiated, and therefore a uniformly hardened coating is not obtained.
The following functions and characteristics are also desired in sheets used for coating objects.
(1) The sheet can provide rustproofing protection to a metallic substrate when the sheet is applied on the substrate.
(2) The desired gloss can be imparted to the sheet surface.
Hitherto, methods have been known whereby the gloss on the sheet surface is modified by transferring the desired irregularities from process paper to the said sheet surface. However, in this method, when the sheet is hardened by heating, the resin of the surface portion of the sheet is melted, and therefore a sheet with the desired gloss is difficult to obtain.
(3) Photochromic properties can be imparted to the sheet.
Japanese Patent Publication No. 57-10913 discloses an adhesive coating consisting of a plastic coating with a layer of an adhesive agent containing a photochromic compound. However, because this adhesive coating is based upon a coating of soft plastic, the surface of the sheet is of low hardness and is easily scratched.
(4) The sheet is not prone to having dust adhere to it; or, the sheet is electrically conductive.
Electrically conductive coating materials have been known for some time. For example, there are antistatic agents applied to plastic or metal articles, sealers used for electroplated objects, coatings for electronic materials used for electrodes, volumes, condensers, and printing of circuit boards, and others.
Japanese Laid-Open Patent Publication No. 60-219270 discloses a type of coating material or sheet that contains a binder and an electrically conductive powder, and is hardened by light, other radiation, electron beams, etc. By use of this type of coating material or sheet, coatings with high strength and hardness as well as excellent electrical conductivity and transparency can be formed.
However, when the coating materials disclosed in this patent publication are used, solvents are required, and therefore, as previously mentioned, the working environment deteriorates, or a long time is necessary for the removal of the solvent. On the other hand, if the said type of sheet is used, then, because these sheets are not flexible, application to substrates with curved surfaces is difficult.
(5) Dew should not condense on the sheet surface.
Japanese Patent Publication No. 62-205171 proposes a type of coating material prepared by formulating a highly water-absorbant resin in a hydrophilic or oilphilic solvent. However, the applied coating material should be dried to remove the solvent, and therefore, the working environment deteriorates, or the removal of the solvent prolongs the processing time.
(6) After hardening, the surface of the sheet should resist soiling. (7) The sheet should have an attractive external appearance and excellent physical properties. Japanese Laid-Open Patent Publication No. 62-16930 proposes a type of colored coating with a attractive external appearance and excellent weather resistance, fabricated by laminating a colored layer containing an unsaturated polyester polyol and a transparent layer composed of a cross-linked polymer composition.
Because the transparent layer is laminated onto the surface of a colored layer in this type of colored coating, the external appearance of the coating gives an impression of depth. However, the colored film is prepared by applying a colored plastic layer onto the surface of a hardened transparent plastic layer, so the coating lacks ductility and malleability. Consequently, wrinkles are liable to form when this coating is used to cover a substrate with an irregular surface.
Japanese Laid-Open Patent Publication No. 62-271735 discloses a sheet obtained by consecutive lamination of a surface layer composed of a thermoplastic resin composition and a colored layer composed of a crosslinking polymer composition containing metallic powder and/or a coloring agent onto a smooth release sheet with mold-releasing characteristics. However, this type of sheet also lacks extensibility.
Japanese Laid-Open Patent Publication No. 58-57472 proposes a type of sheet with a surface layer principally composed of semihardened resin and a base layer principally composed of hot melt resin. Adhesiveness is imparted to the base layer of this sheet by heating. However, heating is required when the sheet is applied to the surface of the substrate, so working characteristics are poor. Moreover, raising the temperature of an object that has been covered with such a sheet causes peeling at the margin of the sheet.